1. Field of the Invention
The present invention relates to a method of manufacturing a tubular hollow body with increased diameter portions which are arranged at a distance from each other, wherein the method uses the internal high pressure deformation process, and to a device for carrying out the method.
2. Description of the Related Art
It is known in the art to shape or deform tubular hollow bodies by means of the internal high pressure deforming or shaping process. For example, EP 0 439 764 B1 describes a method of manufacturing hollow single-part work pieces of metal. In this method, an initially tubular hollow body is widened under the pressure of a liquid while simultaneously axially feeding or forwardly moving the hollow body and pressing the hollow body against the die of a divided shaping tool.
The length of the hollow body to be deformed is limited because the free buckling length increases with the length of the hollow body and axial feeding without simultaneous increase of the internal pressure would lead to the formation of creases in the hollow body. In addition, when increasing the internal pressure, the holding force necessary in the case of longer lengths increases significantly. This, in turn, results in increased manufacturing costs. Accordingly, this known method has its limits, particularly when more than two increased diameter portions are to be manufactured in one work step.
In addition, U.S. Pat. No. 4,788,843 discloses a method and a device for hydraulically shaping a tubular hollow body in which more than two bulges or increased diameter portions can be produced in a hollow body in one work step. For this purpose, the hollow body is placed in a shaping tool, is secured in its position by means of mandrels at the ends of the hollow body and is sealed. Fluid is subsequently filled into the hollow body through the mandrels at the ends. Subsequently, pressing pistons are pushed through the mandrels into the hollow body, so that the required hydraulic internal pressure is built up and the increased diameter portions are formed. U-shaped hollow bodies can also be processed in th is manner.
However, independently of whether the hollow body is straight or curved, this method has the disadvantage that the wall thickness of the hollow body inevitably is decreased in the areas of the radially expanded portions. Consequently, this method can only be used for manufacturing those products in which the requirements with respect to accuracy to size and strength in the deformed areas is of minor importance.
Finally, when using this method, the length of the hollow body which can be processed is limited.